Project Summary Structural Basis of APOBEC Functions and Interactions with HIV-Vif The APOBEC (Apolioprotein B mRNA-editing Enzyme Catalytic polypeptide) family of cytidine deaminases deaminate are found only in vertebrates and all APOBEC3 subfamily proteins are found only in primates. By deaminating the cytidine to cause mutation to uridine on DNA/RNA, APOBEC enzymes achieve remarkably diverse cellular functions through specific targeting to the intented ssDNA or RNA through a combination of regulations including spatial and temporal and substrate specificity. For example, APOBEC1 (A1) specifically modigy the mRNA of a protein that play a role in cholestoral metabolism; AID, another member of APOBEC family, is required for antibody maturation process including somatic hypermutation and recombination class switch; APOBEC2 (A2) is involved in cardiac and skeletal muscle development; and A3 proteins, a subfamily contains seven members (AA-H), can restrict foreign and internal nucleic acids that poses danger to the genome integrity, which include internal retroelements and transposons as well as external retroviruses and otherinfectious viral pathogens, such as Human Immunodeficiency Virus (HIV), Hepatitis B Virus (HBV) . For retroviruses like HIV viruses to overcome the anti-HIV activity of APOBEC enyzmes, they encode a protein called Vif (virus virulent factor) that specifically bind to and inactivate APOBEC enzymes through unbuquitination degradation parthway. Even though their deamination activity is the key to excercute their biological functions, APOBEC enzymes can accidental mutations when proper regulations are not in place, which could lead to human deseases such as immune difficiency and cancer. Our long- term goals are to understand the structural/functional relationship for APOBEC cellular function and their anti-viral activity. Our specific aims are to understand the structural basis of APOBEC's functions and and the mechanisms that underlie substrate specificity and anti-HIV and anti-viral activities, with particular focuses on the APOBEC3 subfamily members that have strong anti- retroelements and anti-HIV activities. The outcome of this proposed research will provide valuable information for understanding the molecular details of the APOBEC enzyme family and the mechanisms of substrate specificity, which can be used for the potential drug development to provide therapy for HIV/AIDS, immune disorders, and other diseases such as cancer.